At least one grid can be disposed between an anode and a cathode of an x-ray tube for improved electron beam control and for a smaller electron beam spot size, and a resulting smaller x-ray spot size. The grid can have a voltage that is different from a voltage of an electron emitter on the cathode. If two grids are used, one grid can have a voltage that is more positive than the voltage of the electron emitter and the other grid can have a voltage that is less positive than the voltage of the electron emitter. The electron emitter can have a very large absolute value of voltage, such as negative tens of kilovolts for example. Voltage for the electron emitter can be provided by a primary high voltage multiplier (“primary HVM”) and a grid high voltage multiplier (“grid HVM”).
One method to provide voltage to the grid(s) is to use an alternating current source, which can be connected to ground at one end. The alternating current source can provide alternating current to the grid HVM. An input to the grid HVM can be electrically connected to the primary HVM. The grid HVM can then generate a voltage for the grid that is more positive or less positive than the voltage provided by the HVM. For example, the primary HVM might provide negative 40 kV, a grid may generate a negative 500 volts, thus providing negative 40.5 kV to a grid. If there is a second grid HVM, it may be configured to generate a positive voltage, such as positive 500 volts for example, thus providing negative 39.5 kV to a second grid. Typically, voltage to each grid may be controlled. Typically only one grid at a time would be used.
A problem of the previous design is a very large voltage differential between the alternating current source and the grid HVM. The alternating current source might provide an alternating current having an average value of zero or near zero volts. The alternating current source can transfer this alternating current signal, through a transformer, to the grid HVM, which has a very large DC bias, such as negative 40 kilovolts for example.
In order to prevent arcing between the alternating current source and the grid HVM, special precautions may be needed, such as a large amount of insulation on transformer primary and secondary wires, or other voltage standoff methods. This added insulation or other voltage standoff methods can result in an increased power supply size and weight, which can be undesirable. Also, the increased insulation or other voltage standoff methods can result in power transfer inefficiencies, thus resulting in wasted electrical power. Power supply size, weight, and power loss are especially significant for portable x-ray sources. Furthermore, the large voltage difference between the grid HVM and the alternating current source (e.g. tens of kilovolts), can result in failures due to arcing, in spite of added insulation, because it is difficult to standoff such large voltages without an occasional failure.